Wake-on-lan masquerade

OK, the title of this post was the best I could think of, but it's not exactly right. I'm hoping someone can help me with a simple-in-concept but possibly difficult (or even impossible, though I doubt it) to set up idea.

What I would like to do is set up one small, low power server (called minnie) to run alongside my normal, fairly heavyweight do-everything server (mickey, notice the mouse theme? ) which consumes a fair whack of power even on idle. Minnie will do the majority of the day-to-day simple tasks (email/routing etc.), allowing me to put mickey into STR sleep, then wake it up when it is needed.

The problem here is that I would like the mickey to always appear online. What I want is for Minnie to pretend to be mickey while he is asleep, then when a request comes in for mickey, she wakes him up and allows him to do it. I do NOT want minnie to sit between mickey and the network (physically or logically), just to pretend to be him until he is needed, wake him, then stop pretending till she sends him back to sleep.

I can post more details if required, but as a simple example, mickey is a file server. I want to be able to mount a share (on NFS or SMB), and him only wake up when I request a file. Preferably Minnie would cache some files, but this is not a major requirement and would increase complexity.

So... Any ideas? My own idea were heading towards a virtual interface sitting on minnie's network card, using mickeys IP, which monitored for someone accessing it, at which point it woke mickey up and brought the virtual interface down, but I don't think this would work as I expect.